1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly to a liquid crystal display of a direct view type.
2. Description of the Background Art
Recently, a liquid crystal display is actively used for a personal OA (Office Automation) instrument such as a word processor, notebook personal computer or desktop personal computer as well as an image display unit such as a television because of its small thickness, lightweight and small power consumption. Particularly, a liquid crystal display of an active matrix type is actively developed as a portable display because it enables a high resolution display in addition to the advantage of small thickness, lightweight and small power consumption. (Hisao Ishii, Denshi-Gijutsu (Electrotechnology), July 1997, p.7)
A liquid crystal itself does not emit light. It functions as a display by controlling light which is transmitted therethrough. There are generally two ways of directing light through the liquid crystal. One is related to a liquid crystal of a direct view or transmission type in which a back light is provided behind the liquid crystal, and the other is related to a liquid crystal of a reflective type which allows light in the environment surrounding a viewer to be transmitted through the liquid crystal from front, reflected by a reflector plate behind the liquid crystal and again transmitted through the liquid crystal from back.
The liquid crystal display of the direct view type is always provided with the back light of a light source behind the liquid crystal panel. As shown in FIGS. 7A and 7B, there are two types of back lights: edge light type and vertical type. The edge light type shown in FIG. 7B includes: a bar like light 51 (which is either a cold cathode tube or hot cathode tube) at an end of a light conducting plate 54; a reflector plate 53 behind light conducting plate 54; and a scattering or diffusion plate 52 in front of light conducting plate 54 which has a lens sheet (that is, an optical sheet such as a prism sheet and wave sheet) and is provided with a protection function. The vertical type shown in FIG. 7A has a structure having light 51 immediately below diffusion plate 52, and it does not require the light conducting plate used in the edge light type (see for example, Keiichi Nakajima, Denshi-Gijutsu (Electrotechnology), July 1997, p.11).
The above described liquid crystal display does not have a large volume nor weight unlike a CRT(Cathode-Ray Tube). In addition, it needs not be operated at a high voltage, so that power consumption thereof is small. In other words, the liquid crystal display is characterized by its small thickness, lightweight and small power consumption unlike the CRT. However, transmittance of a polarizer for a liquid crystal panel is about 40% to 45%, and half of the light from the back light which consumes the largest amount of power in the liquid crystal display is not effectively utilized. This is because only polarization component in one direction is transmitted of all light which is directed to the polarizer from the back light, and that in a direction which is orthogonal to the above mentioned direction is absorbed by the polarizer.
To solve this problem, a method of inserting a so called reflecting-polarizer or a highly transmissive polarizer between the liquid crystal panel and the back light has been proposed, and a product manufactured in accordance with the method has been obtained. Such products include DBEF (Double Brightness Enhancement Film) of 3M Corporation and Transmax of Merck & Co., Inc. Such reflecting-polarizer is obtained by forming a multilayer film having an optical function on a polymer film by an evaporation method, or by specifically aligning molecules of a cholesteric liquid crystal for application on a polymer film. As shown in FIG. 8, such reflecting-polarizer allows transmission of only one of two polarization components from the back light (that is, two linearly polarized light which are orthogonal to each other or two circularly polarized light which rotate in opposite directions) and reflects the other. As the reflection is mirror reflection, the reflecting-polarizer looks like a mirror.
The polarized light which has been reflected by the reflecting-polarizer is returned to and reflected by the back light, so that the completely polarized state is partially cancelled. In other words, light having one polarization component is converted to light having two polarization components and again directed to the reflecting-polarizer. Again, one of the polarization components is transmitted through the reflecting-polarizer and the other reflected thereby. The process is repeated. Therefore, a larger amount of light is directed from the back light through the reflecting-polarizer, and therefore light is effectively utilized. As a result, even when brightness of the back light is reduced, high brightness is obtained for the liquid crystal display. In addition, as a back light which consumes a small amount of power can be applied, overall power consumption of the liquid crystal display is reduced.
However, the function of the reflecting-polarizer which allows transmission of only one polarization component and reflection of the other is insufficient. Further, as light is absorbed by various optical components during travel, utilizing efficiency of the light would not be so high as compared with the conventional case.
Besides the above mentioned reflecting-polarizer, referring to FIG. 9, a method of using an anisotropic scatterer which allows transmission of polarization component in one direction and forward scattering of that in a direction which is orthogonal thereto has been proposed (Japanese Patent Laying-Open No. 8-76114). The scattering by the anisotropic scatterer is mainly forward scattering. It is thus different from a scattering-polarizer which is used in the present invention and will later be described. The principle of forward scattering by the anisotropic scatterer is still unclear in many respects.
In a conventional liquid crystal display in which the reflecting-polarizer is held between a back light and a liquid crystal panel, light from the back light is effectively utilized. Thus, brightness of the back light is reduced as compared with the conventional display, whereby power consumption of the liquid crystal display can be reduced. However, disadvantageously, the reflecting-polarizer is expensive as it is manufactured with a complicated manufacturing process and requires high technology for aligning molecules of a liquid crystal. In addition, a material of the reflecting-polarizer may readily allow deformation due to heat and coloring, thereby reducing quality of display. Further, the anisotropic scatterer which allows transmission of light component in one direction and forward scattering of that in the direction orthogonal thereto had a problem which is similar to that of the reflecting-polarizer.